Millions of once-faithful BlackBerry (NASDAQ: BBRY) users have made a switch to an iPhone device over the past few years. T-Mobile's (NASDAQ: TMUS) recent advertisement is targeting the remaining faithful users, and its irking BlackBerry CEO John Chen.

T-Mobile is offering Apple's (NASDAQ: AAPL) iPhone 5S for $0 down. To promote the sale, the company ran an ad poster that said, “Get the tools to do more. $0 down on iPhone 5S. GREAT OFFER for BlackBerry customers.”

“As we were never told of their plans in advance, I can only guess that T-Mobile thought its great offer for BlackBerry customers' would be well received,” Chen wrote in a blog post on the company's website. “T-Mobile could not have been more wrong.”

To customers, Chen said: “I can assure you that we are outraged too.” Many BlackBerry faithfuls took to Twitter to express their frustration, which prompted T-Mobile's CEO John Legere to apologize.

A T-Mobile spokesman told Bloomberg, “We are happy to be a BlackBerry partner and apologize for any confusion.” T-Mobile also told Bloomberg that it will start providing free, expedited shipping of BlackBerry devices by the end of this week, “as an additional courtesy to our BlackBerry customers.”